1. Field of the Invention
The invention relates in general to methods and circuits for sensing and correcting skew between the two signals in a differential lane.
2. Description of Related Art
In high-speed digital applications it is common to use differential inputs and outputs to interconnect integrated circuits at the next higher level of integration (e.g. circuit board or multi-chip module). The reason for this is that differential signaling offers relative immunity to noise and drift that would otherwise have negative impact on the signal integrity of the lane carrying the high-speed information.
It is, however, known that this relative immunity to noise and drift can be compromised by skew. The reason for this is that when the skew between the differential signals is sufficiently large, the receiving stage may switch as a result of the transition of (substantially) one line of the differential pair. In such a case, the drift and noise canceling features of differential signaling are nearly eliminated.
U.S. Pat. Nos. 6,812,777 and 6,963,237 issued Nov. 2, 2004 and Nov. 8, 2005, respectively, to Tamura et al describe a method and circuits for controlling the differential skew of an output circuit. This method is illustrated in his FIGS. 19 and 20 , included here as FIGS. 1 and 2, by example. Tamura adjusts the duty cycle of the output signals to eliminate the differential skew. Tamura, however, assumes that the differential skew arises as a consequence of problems in the driver circuit or the output circuit that is the subject of the patent. Tamura's invention does not deal with skew introduced by interconnect between the output circuit he has de-skewed and the input circuit of another, remote, integrated circuit.
U.S. Pat. No. 6,686,779 issued Feb. 3, 2004 to Takefumi Yoshikawa describes a method for controlling the differential skew of an output circuit. Yoshikawa uses independently programmable pull down capability for the true and complement versions of the differential output, thereby providing an ability to de-skew the output circuit. Yoshikawa, however, assumes that the differential skew arises as a consequence of problems in the driver circuit or the output circuit that is the subject of the patent. Yoshikawa's invention does not deal with skew introduced by interconnect between the output circuit he has de-skewed and the input circuit of another, remote, integrated circuit.
U.S. Pat. No. 6,909,980 issued Jun. 21, 2005 to Chenjing Fernando describes a method for deciding how an article of test equipment, e.g. an oscilloscope, should adjust the timing of input differential signals to obtain optimal skew values in an eye diagram. Fernando uses independently programmable “paired independent skew circuits” for the true and complement versions of a differential input, thereby providing an ability to de-skew the signal circuit. Fernando, however, assumes that the differential skew arises as a consequence of problems in the interconnection to the test equipment and that is the subject of the patent. Fernando's invention does not deal with skew introduced by interconnect between a source and the input circuit of another, remote, integrated circuit that is not a part of the test equipment that is the subject of his invention.
In contrast to the prior art cited in the patents by Tamura et al, Yoshikawa, and Fernando, the invention described here is a method for sensing skew at the receiving end of a differential signaling lane and automatically eliminating it independently of whether it was caused by problems in the driver IC, the interconnect, or the receiver input impedance.
The circuit driving the transmitter differential signals out of a transmitter IC may introduce skew because of asymmetric driving capability or other defects such as asymmetric loading of the output or of the input of the driver. But for the most part, skew is introduced by differences in the length, or effective length, of interconnects. In the construction of media for interconnecting integrated circuits there are a variety of opportunities for inadvertently introducing skew. If a pair of differential lines have to be routed between a transmitter and a receiver, it can at times be very difficult to ensure that the two lines have the same effective length because of length differences or differences in bends and corners in the two transmission paths, because the lines may have to be run in different layers of interconnect or may have spatial variations in dielectric constant, and because of non-homogeneous transmission media like glass fiber based laminates. A secondary, though still significant, source of skew is the difference in the impedance and/or frequency response of the two paths in a differential lane. Such differences can arise because of the differences in parasitics loading the paths caused by proximity of components, vias, and co-planar grounds, among others.